The dinosaurs never saw it coming. The first dinosaurs appeared on Earth about 230 million years ago. Dinosaurs (the name means "terrible lizards") ruled the planet for nearly 165 million years ago, they all disappeared. Along with the dinos, about 50 percent of all planet's plants and animals were wiped out in a mass extinction. An extinction is when entire species dies out.

What caused this huge wipeout? Some scientists once thought dinosaurs got too big to survive. Others believed dino eggs were eaten by mammals. Now, scientists have come up with other possible reasons.

The Big Crash

A comet or asteroid about six miles across may have slammed into the Yucatan, in Southern Mexico, 65 million years ago. The blast sent clouds of dust into Earth's atmosphere. Trillions of tons of burning ash from the blast may also have set fire to forests, creating clouds of smoke. The dust and smoke blocked the sunlight for years and so temperatures dropped. The result: global cooling that did away with the dinos.

Blowing Their Tops

Before the dinos died out, huge volcanic eruptions took place in Western India. They covered half of modern-day India with a layer of volcanic rock one mile thick. Clouds of ash and dust could have blotted out the sun, lowering the planet's temperatures. Volcanic gases may have produced acid rain, which hurt plant and sea life. These disasters could have led to mass extinction.

Bad Bugs

As the planet's temperature rose 65 million years ago, it became a perfect breeding ground for germs and insects. The insects could have spread new diseases that dinosaurs and other animals couldn't fight off.

Sea This

Scientists believe that Earth's oceans were once much higher than they are today. Then, millions of years ago, the continents rose and sea levels dropped. Many once-wet areas became cool and dry. During that period, for example, North America was divided by a sea called the Interior Seaway drained. Not only did many sea animals die, but the dry land could no longer produce the food that dinosaurs needed to survive. This may have led to their dying out.

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Factoid

Among the animals that survived the extinction of the dinosaurs 65 million years ago were crocodiles and alligators. In fact, their body shapes haven't changed much since that time long ago.

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Here Today...

Some scientists think a mass extinction may be going right now. They estimated that 30,000 species disappear each year. That's three species an hour. By the end of this century, half of today's species of plants and animals could disappear. The main cause for this: humans.

A species is endangered when it is in immediate danger of becoming extinct. A threatened species is likely to become endangered in the future. Once a species is extinct, it will never come back. Here is just a handful of animal species that are endangered or threatened.